1. Field of the Invention
The present invention relates to a method of estimating a position (e.g., magnetic pole position) of a rotor in a synchronous motor wherein permanent magnets are used for the rotor.
2. Description of Related Art
In a synchronous motor such as a surface permanent magnetic synchronous motor (SPMSM) or an internal permanent magnetic synchronous motor (IPMSM), permanent magnets are used for a rotor, and S and N magnetic poles formed by the permanent magnets are alternately disposed around a shaft of the rotor. During the operation of this motor, an alternating current voltage is applied to windings of a stator, and an alternating current generated from the applied voltage flows through the windings. A phase of the voltage is controlled in synchronization with a position (e.g., magnetic pole position) of a rotor. Therefore, a position detector for detecting the magnetic pole position of the rotor is required. As this detector, a Hall element, an encoder or a resolver is used. However, it is difficult to downsize a motor with a position detector, and it is required to connect the detector with a controller for controlling the motor through connection lines.
Therefore, to control a synchronous motor without using any position detector, a magnetic pole position of a rotor in a motor has recently been estimated by using an induced voltage of the motor. During the operation of the motor, the motor itself induces a voltage changed with the magnetic pole position. Therefore, the magnetic pole position can be estimated from the induced voltage. For example, Published Japanese Patent First Publication No. 2001-251889 discloses a motor model approximated on a dq rotational coordinates system having a d-axis and a q-axis to estimate a magnetic pole position based on an induced voltage. The d-axis is set so as to be directed from an S magnetic pole to an N magnetic pole in a rotor. The q-axis is set to be orthogonal to the d-axis on a plane perpendicular to a rotation axis of the rotor. An original point of the coordinates system is placed on the rotation axis.
In the estimation based on this motor model, a shaft shift Δθ of an estimated pole position from a true pole position (i.e., d-axis) in a synchronous motor is calculated from a component Idc (hereinafter, called d-axis component) of a detected direct current along the d-axis, a component Iqc (hereinafter, called q-axis component) of the detected direct current along the q-axis, a d-axis component V*d of a voltage applied to the motor, a q-axis component V*q of the applied voltage and an instructed rotational speed ω*r of the rotor. An estimated magnetic pole position of the rotor is calculated from the shift Δθ.
However, in an actual motor, an alternating current voltage of three phases is applied to windings of a stator of a motor, and a three-phase alternating current flowing through the windings of the stator is detected. To obtain the components Idc and Iqc of the direct current expressed on the d-q rotational coordinates system, it is required to transform three-dimensional coordinates system for the actually detected alternating current into two-dimensional rotational coordinates system, based on an estimated magnetic pole position. In this case, the components Idc and Iqc include error derived from position error in the estimation of the position. Because a next magnetic pole position is estimated based on the components Idc and Iqc including error, precision of the estimation is inevitably lowered. As a result, when the motor is controlled according to the estimated position, the motor cannot stably be operated.
To solve this problem, direct estimation of a magnetic pole position based on a motor model using an extended induced voltage has been proposed in a paper of “An Extended E.m.f Observer for Salient-Pole Brushless DC Motor's Sensorless Control” in the national meeting in 1999, the Institute of Electrical Engineer of Japan (IEEJ), No. 1026.
In this paper, a magnetic field observer is constituted based on the model defined on an αβ stationary coordinates system representing a two-phase alternating current coordinates system. Each of an alternating current voltage applied to a synchronous motor and an alternating current detected from the motor is expressed by a vector having two components along α- and β-axes of the αβ coordinates system. When giving, to the observer, α- and β-axes components of the applied alternating current voltage and α- and β-axes components of the detected alternating current, a magnetic pole position can be directly estimated. This position is expressed as an angle between the α-axis and a line connecting an original point and a magnetic pole of a rotor in the αβ coordinates system.
In this paper, precision of the estimation of a magnetic pole position can be improved by the motor model using the alternating current voltage and the alternating current defined on the two-phase alternating current coordinates system. However, because the model includes a differential term to calculate an induced voltage in detail, an amount of calculation required for the estimation is enormously increased.